This invention relates to microcircuit packages and more particularly to all-metal flat packages for microcircuits.
All-metal flat packages normally include three major metal components. The first component is a frame which generally is a continuous ring of metal that extends around the periphery of the package and which forms the side walls of the package. Electrical leads extending through the frame constitute the second component of the flat package. Most often the leads pass through two opposing sides of the package and are normally glass-sealed within holes in the frame. The third component of the package is the bottom upon which the microcircuit substrate is affixed. In addition to these three components, there is, of course, a lid which is attached after the microcircuit has been installed in the package.
Normally all-metal microcircuit packages which are flat packages are made of Kovar, with the Kovar frame and bottom often being joined together by a high temperature copper braze. Kovar is a trademark of the Westinghouse Corporation for an iron-nickel-cobalt alloy (29% nickel, 17% cobalt, 53% iron, and 1% minor ingredients). Indeed, Kovar flat packages comprise a great majority of the all-metal flat packages made in the world. Kovar is the likely choice for making all-metal flat packages since leads readily can be glass sealed in Kovar and since its coefficient of thermal expansion closely matches 96% alumina, the material normally used for microcircuit substrates which are housed in the packages. Since the coefficient of expansion of Kovar closely approximates that of alumina, the alumina substrate may be soldered to the Kovar package. While Kovar has good glass-sealing and thermal expansion properties, it has a very low coefficient of thermal conductivity, about 0.04 Cal/Cm.sup.2 /Cm/Sec/.degree.C. This presents a problem of heat dissipation from power chips through the bottom of the flat package to a heat sink. About the only practical way to reduce the thermal impedence of an all-Kovar flat package is to employ a thin bottom, thereby reducing the length of the heat path from the substrate of the microcircuit chip to the heat sink. Such technique, however, reduces the strength of the package bottom.
Stainless steel or cold rolled steel have occasionally been used for flat packages. Leads readily can be compression glass-sealed in the steel frames but, since the coefficients of thermal expansion of stainless steel or cold rolled steel are considerably greater than 96% alumina, the alumina substrates are usually attached to the bottom of the flat package with an adhesive such as an epoxy. Unfortunately, attachment with adhesive tends to be an impediment to efficient heat transfer. Like Kovar, stainless or cold rolled steel possess very low coefficients of thermal conductivity making heat dissipation a problem. Moreover, soldering alumina substrates to steel is often not a feasible alternative to increase thermal conductivity. It has been found that if there is a significant mismatch in thermal expansion between the microcircuit substrate and the flat package bottom, and the substrate is soldered in place, the substrate will crack during thermal cycling and the thermal shock. For example, a span of more than about one-half inch of 96% alumina substrate when soldered into place on cold rolled steel normally will fail. Most microcircuits of the hybrid variety which dissipate a considerable amount of power utilize substrates significantly larger than one-half inch.
In an effort to accommodate power dissipation from substrates mounted in steel packages, some manufactures having divided the circuit into components. The power dissipation portion of the circuit, for example, is mounted on a beryllia pad and the remaining portion of the circuit may be attached to an alumina substrate. Beryllia is more thermally conductive than alumina and has a higher coefficient of expansion than alumina. The beryllia pad, since it carries only the power chip, may be relatively small, is generally round, and can be soldered to cold rolled steel to improve thermal conductivity. The remaining portion of the circuit, attached to an alumina substrate, does not dissipate substantial amounts of heat and can be positioned with epoxy.